This invention relates to a process for producing textile-like non-woven sheet material and more particularly to a process for forming uniform binderless webs of synthetic fibers from a water dispersion using a conventional moving wire screen.
Though dry-laid and spun-bonded non-wovens made of synthetic fibers are known, heretofore, it has been difficult to form uniform water-laid webs from non-fibrillatable man made fibers, particularly those which are hydrophobic, have a low denier, and are relatively long (i.e., have a length greater than about 1/4 inch and a length to diameter ratio of between about 400:1 and 700:1) or have round or non-flat surface characteristics. Crimped fibers are also particularly difficult to form into uniform water-laid webs.
The difficulty in forming such webs from liquid phase dispersions lies principally in achieving good web formation on a moving wire screen and in transferring the web from the screen without its being torn or pulled apart.
Uniform web formation is hard to obtain, because it is difficult to disperse relatively long man made non-fibrillatable fibers in water and hard to maintain them well dispersed without undesirable flocculations. It is also difficult to form such webs uniformly, because water drains through them so quickly it is not possible to use shake mechanisms such as are common in the making of paper for achieving uniform formation.
It is known that in order to disperse fibers in water, each fiber has to be well wetted with the water. In hydrophilic fibers, such as viscose rayon, acetate rayon, cellulose acetate & polyvinyl acetate, wetting the fibers is not much of a problem. Wetting hydrophobic fibers made from polymers such as polyamides, polyesters, polyolefins and the like, however, is a problem since such fibers do not wet easily. Thus, either a wetting agent in the water or some surface coating or pretreatment of a wetting agent on the fibers, or both, is necessary. However, since most wetting agents, or surfactants, are also good foam generating agents, when present in amounts adequate to wet the fibers satisfactorily, they tend to create copious quantities of unwanted foam even under gentle agitation conditions. This in turn tends to float the fibers out of the dispersion.
Further, we have found that, when defoaming agents are added to dispersions of such fibers, the fibers tend to flocculate making the formation of a uniform web more difficult.
It is also known to use wetting agents, in wetlaying systems, which are not good foam generating agents, to disperse non-fibrillatable short synthetic fibers of 10 mm or less in length and having a length to diameter ratio within a range of from about 3:1 to 10:1. However, even with intense agitation such agents have not been able to satisfactorily disperse longer synethic fibers with length to diameter ratios of between about 400:1 and 700:1.
In addition, it has been observed that, when the viscosity of the water is increased in a water dispersion system the rate of fiber reflocculation is reduced, thus improving web or sheet formation. Thickeners such as natural and synthetic gums have been used as viscosity modifiers to raise the viscosity of water for this purpose.
To increase initial wet web strength, in some instances, hydrated (fibrillated) wood or other natural fibers and even fibrillated synthetic fibers have been used in non-fibrillatable synthetic fiber furnishes to hold the web together while being transferred from a moving forming wire across unsupported draws to wet presses or other treating equipment where a binder is added to hold the fibers together more permanently. In such a web, before the addition of any adhesive, the web is held together in part by the mechanical interlocking of the fibrillated fibers.
Other techniques for increasing the initial wet strength of man made non-fibrillatable fiber sheet materials include coating or encapsulating short natural fibers with latex polymer binders precipitated on the fibers in the water before forming the sheet. These binders hold the sheet together and allow its continuous removal from the forming wire without breaking or tearing.
The use of precipitated latex polymers tend to produce softer and more textile like properties in the web than in normal papermaking processes, but it is a more expensive technique and has the added disadvantage of being tacky, thus making it difficult to maintain clean and non-tacky machine conditions. Further, the use of fibrillated natural or synthetic fibers as part of the furnish, though advantageous in the production of synthetic paper, is not desired in wet-laid non-wovens for textile use. These fibers tend to make the material stiff and non-porous, properties which are not desired in non-wovens intended for use as replacement fabrics for textiles.
One additional technique, namely that disclosed in U.S. Pat. No. 3,223,581, has also been used to hold paperlike sheets of synthetic polymer fibers together. This technique involves no binders, but a very careful control of the amount of water in the web as it is transferred from the forming wire. It also requires that the fibers forming the web have essentially smooth and flat or planar surfaces to provide large areas of surface contact between the fibers. Only with these conditions does the invention of that disclosure indicate how to give the web sufficient initial wet strength to hold together without binder material while being transferred from the forming machine without being torn or pulled apart. One disadvantage of the process of the U.S. Pat. No. 3,223,581 is that it is limited to using fibers having essentially smooth, flat surfaces for providing large area surface contact between the fibers forming the sheet. In addition, such fibers produce relatively dense, stiff and "papery" sheets which are undesirable in non-wovens for textile use. Round and other fibers having no flat surfaces will not work in that process. They are, however, usable in the process according to the present invention.
Further, although it is possible to disperse non-fibrillatable hydrophobic synthetic fibers as disclosed in U.S. Pat. No. 3,007,840 and British Patent No. 1,129,757 by means of a substantially liquid phase free thixotropic foam as the dispersing medium and to form a web from the dispersion, no one, heretofore, has been able to accomplish satisfactory dispersion and web formation of such fibers using conventional non-woven water-laying techniques.
It is therefore one object of this invention to provide a new and improved method for producing non-woven sheet material webs suitable for use in textiles or as textile replacements.
Another object of the invention is to provide a novel process for forming such material from man made nonfibrillatable fibers or from a mixture of such fibers with non-fibrillated natural or synthetic pulp or flock fibers.
Still another object of the invention is to provide a method for forming such material of fibers having virtually no flat surfaces from an aqueous slurry by conventional wetlaid non-woven web forming techniques.
Yet another object of the invention is to provide webs which are self supporting in the wet or never dried state (i.e., with enough initial wet web strength to enable the web to be drawn off the forming wire and through conventional wet pressing and treatment equipment) from synthetic fibers having no large flat surface areas and without using either fibrillated fibers in the furnish or binders at the wet end to hold the web together.
A further object of the invention is to provide a method for forming such sheet material using a novel method for uniformly dispersing the fibers in water.
A still further object of the invention is to provide a method having the above characteristics in which the already dispersed fibers are substantially prevented from reflocculating.
Further, other and additional objects and advantages of the invention will become apparent from the summary and detailed description which follows, as well as from the appended claims.